1. Field of the Invention
This invention relates generally to a method and apparatus for controlling an internal combustion engine, and more particularly to a microprocessor-based electronic engine control system having a memory preprogrammed with various control laws and control schedules responsive to one or more sensed engine-operating parameters for generating signals for controlling fuel injection, ignition timing, EGR control, or the like.
2. Statement of the Prior Art
Many of the patents of the prior art recognize the need for employing the enhanced accuracy of digital control systems for more accurately controlling one or more functions of an internal combustion engine.
U.S. Pat. No. 3,969,614 which issued to David F. Moyer, et al on July 13, 1976 is typical of such systems as are U.S. Pat. No. 3,835,819 which issued to Robert L. Anderson, Jr. On Sept. 17, 1974; U.S. Pat. No. 3,904,856 which issued to Louis Monpetit on Sept. 9, 1975; and U.S. Pat. No. 3,906,207 which issued to Jean-Pierre Rivere, et al on Sept. 16, 1975. All of these Patents represent a break-away from the purely analog control systems of the past, but neither the accuracy, reliability, or number of functions controlled is sufficient to meet present day requirements.
Future internal combustion engines will require that emissions be tightly controlled due to ever-increasing governmental regulations, while fuel consumption is minimized and drivability improved over the entire operating range of the engine. None of the systems of the prior art provide a method and apparatus for controlling the operation of an internal combustion engine over even a portion of its operating range with sufficient accuracy to attain minimal emissions and minimal fuel consumption while simultaneously improving drivability.
The engine control systems of the prior art do not normally employ oxygen feedback systems to control engine-operating functions since the oxygen sensor's impedance varies as a function of its operating temperature and the signal is only valid when the oxygen sensor has reached a pre-determined minimum temperature, for example 300 degrees Centigrade. Therefore, measurement of the sensor's impedance will indicate its cold temperature, however, the impedance at this temperature is normally quite high and previous methods used to measure the impedance of the sensor for test purposes usually masked the sensor operation or sensor output signal during this transition period. As the sensor temperature increases and the impedance falls, the impedance measuring mechanism doesn't mask the sensor signal so it is precisely at the high sensor impedance point and at or near the pre-determined minimum operating temperature that an accurate sensor impedance test signal is desired.
The system of the present invention avoids the problems of the prior art and alleviates the problem of the test impedance masking sensor operation.